This invention relates to an active carbon catalyst for recovering and removing sulfur oxides contained in flue gas after transforming them into sulfuric acid by catalytic oxidation and also to a method of flue gas desulfurization by means of such an active carbon catalyst.
Methods are known for catalytically oxidizing sulfur dioxide gas contained in flue gas in the presence of a catalyst and oxygen at low temperature to eventually turn them into sulfuric acid and recovering the obtained sulfuric acid. Active carbon is the catalyst that is most popularly used with such methods. This is because, if a catalyst comprising ceramic type carriers such as alumina, silica, titania and/or zeolite is used, it does not provide a sufficient level of activity and hence catalytic components such as a metal or a metal oxide have to be carried on it but such catalytic components are prone to be attacked by sulfuric acid generated as reaction product and become dissolved or transformed to lose their catalytic effect so that it is highly difficult to make them stably remain catalytically active for a prolonged period of time. Active carbon, on the other hand, shows a substantive level of activity without carrying catalytic components such as a metal or a metal oxide and the level of activity is maintained for a prolonged period of time so that it is substantially free from the above identified problem.
However, from the viewpoint of using active carbon in a flue gas desulfurization plant running on a commercial basis, commercially available active carbon does not necessarily always maintain a high level of activity and therefore a large volume of active carbon will have to be supplied to constantly achieve the intended desulfurization efficiency. Thus, the use of active carbon will more often than not be costly if compared with other desulfurization processes such as a wet type flue gas desulfurization process. The reason why active carbon cannot maintain a high level of activity is generally believed to be that, while active carbon intrinsically shows a very high level of activity of adsorbing and oxidizing sulfur dioxide gas (hereinafter simply referred to as xe2x80x9cactivityxe2x80x9d), once sulfur dioxide gas is adsorbed by the surface of active carbon and oxidized in the presence of moisture at low temperature, it absorbs moisture to become dilute sulfuric acid, which by turn covers or closes, if partly, the pores of active carbon to interfere with the diffusion of sulfur dioxide gas and the possible contact thereof with the active sites within active carbon so that consequently the active sites within active carbon will not be fully utilized. Thus, there have been proposed various techniques for fully exploiting the high activity level of active carbon by providing active carbon with water repellency so that the generated dilute sulfuric acid may be quickly expelled from the pores of active carbon to maintain the high activity level thereof.
For instance, there is a report in Chem. Eng. Comm. Vol. 60 (1987), p.253 that the rate constant of the reaction of adsorbing and oxidizing sulfur dioxide gas is tripled by spraying a solution of dispersed polytetrafluoroethylene (PTFE) to active carbon having an average grain diameter of 0.78 mm if PTFE is added by 8 to 20%. Japanese Patent Application Laid-Open No. 59-36531 describes that the effect of active carbon of adsorbing and oxidizing sulfur oxide gas is increased by treating active carbon for water repellency and, more specifically, granular active carbon with a grain size of 5 to 10 mm comes to show a remarkably high activity level as catalyst when it is impregnated with a solution of dispersed PTFE and heat treated at 200xc2x0 C. for 2 hours if compared with untreated granular active carbon.
The inventors of the present invention conducted an experiment as described below in order to examine the effectiveness of the above known methods for improving the catalytic activity of active carbon. Firstly, according to the known techniques of providing active carbon with water repellency, commercially available granular active carbon having a grain size between 2.8 and 4.0 mm was made to be impregnated with PTFE by spraying or immersion to find that the activity was improved to a certain extent and retained for a prolonged period of time if compared with untreated active carbon. However, the improvement of activity to such an extent is not enough in view of the competition of a process using treated active carbon with other desulfurization processes to be adopted in a flue gas desulfurization plant running on a commercial basis and the inventors realized that a further improvement has to be achieved for the catalytic activity of active carbon.
As a result of additional research efforts, the inventors of the present invention came to find that the catalytic activity of active carbon can be effectively improved by providing only the macropores (minute pores with a diameter greater than 5nm) of active carbon with water repellency. More specifically, they found that the catalytic activity of granular active carbon is improved to a large extent by making the granular active carbon impregnated with polystyrene (PS) particles having a sphere equivalent diameter between 10 and 100 nm as water-repellent substance. However, when particles of fluororesin such as PTFE that is more water-repellent than PS are used, they cannot successfully make macropores of active carbon water-repellent by a known technique of impregnating active carbon with a water-repellent substance and making it carry the latter such as the spraying or immersion technique because commercially available fluororesin particles have a relatively large diameter of 100 nm or more. In order to make clear this fact, the inventors of the present invention prepared an active carbon catalyst by causing commercially available granular active carbon to be impregnated with and carry PTFE by means of the spraying or immersion technique using a PTFE-dispersed solution and then analyzed the fluorine distribution profile of the prepared catalyst by means of EPMA. As a result of the analysis, it was found that PTFE particles had not got to the inside of the granular active carbon and only remained adhering to the outer surface of the granules of active carbon. More specifically, since commercially available granular active carbon practically does not have pores with a diameter greater than 1 xcexcm, it is highly difficult for PTFE particles with a diameter between 0.2 and 0.4 xcexcm to enter any of the pores of commercially available active carbon. The result of experiment was similar when the PTFE-dispersed solution was replaced by a solution containing PS particles with an average particle diameter of 0.3 xcexcm in a dispersed state. When the two active carbon catalysts containing respectively the two different types of water-repellent particles were used to test the activity, it was found that the one carrying PTFE particles was slightly more active than the one carrying PS particles, although neither of them did not show the expected level of activity.
The inventors of the present invention further looked into the macropore diameter of active carbon that can most improve the activity of active carbon when the latter is processed for water repellency. Firstly, five different specimens of latex (obtained by dispersing PS particles of relatively similar sizes into water by about 10 wt %) with respective average particle diameters of 10, 28, 55, 102 and 300 nm were prepared. Then, they were diluted to different concentrations between 0.1 and 5 wt % and different granular active carbon samples were immersed respectively into the obtained latex specimens and subsequently dried under reduced pressure to produce so many different active carbon catalysts. As a result, it was found that, among the processed active carbon catalysts, those with PS added by about 1 wt % showed the highest activity regardless of the average diameter of PS particles and that those carrying PS with the average diameter of 28 nm or 55 nm were most active but those carrying PS with the average diameter of 10 nm and 102 nm were slightly less active, whereas those having PS with the average diameter of 300 nm were only slightly more active than unprocessed active carbon catalysts. Fractured PS particles of the sample catalysts with five different PS particle diameters were observed by SEM to find that PS particles with the average particle diameter of 55 nm or less had evenly entered to the inside of active carbon grains whereas PS particles with the average particle diameter of 102 nm were found only near the surface of active carbon grains and those with the average particle diameter of 300 nm were found only on the outer surface of active carbon grains. The reason why the active carbon catalysts carrying PS with the average particle diameter of 10 nm were less active than those carrying PS with the average particle diameter of 28 nm or 55 nm may be that very fine PS particles can clog macropores, although this is a mere speculation. Anyhow, the above experiment suggested that macropores with a diameter greater than the smallest diameter that allows PS particles with an average diameter of 28 nm to enter should be processed to make its macropores water-repellent.
On the basis of the above observations, it was confirmed that the activity of a granular active carbon catalyst can be greatly improved by making its macropores water-repellent, that this activation process is effective when active carbon grains are evenly processed to the inside for water repellency and that fluororesin such as PTFE is more effective than PS for improving the activity of active carbon because the former realizes a higher level of water repellency. Thus, the inventors of the present invention got to an idea of crushing granular active carbon to fine particles, mixing them with fluororesin particles and molding the mixture in view of the fact that commercially available fluororesin particles show a relatively large average particle diameter and cannot effectively make granular active carbon water-repellent simply by impregnating the latter with the former and making the latter carry the former. Then, an experiment was conducted by the inventors of the invention to make both the inter-particulate gaps of powdery active carbon particles (which may be referred to as xe2x80x9clarge macroporesxe2x80x9d) of the molded product and part of the macropores of the original active carbon water-repellent by means of fluororesin particles. The obtained active carbon catalyst showed a level of activity much higher than both the original active carbon and any active carbon catalysts prepared by impregnating them with and making them carry PS particles.
While the inventors of the present invention used to believe about the conditions under which active carbon is crushed and mixed with fluororesin for molding that the inter-particulate gaps of powdery active carbon particles will be modified to a large extent by PTFE to improve the activity thereof simply by crushing active carbon to fine particles as far as possible and mixing them with a PTFE-dispersed solution. Thus, firstly, they crushed commercially available active carbon to particles with an average particle diameter of 10 xcexcm and mixed them with a PTFE-dispersed solution to prepare an active carbon catalyst, which was subsequently evaluated for catalytic activity. However, no expected improvement was obtained in the activity when PTFE was added at a varying rate between 2 and 30 wt %. The reason for this was assumed to be that, when active carbon is crushed too far, the inter-particulate gaps of powdery active carbon particles that provide discharge paths for the produced sulfuric acid are extremely narrowed and then totally clogged by PTFE particles. Thus, the rate of adding PTFE was held constant and the average particle diameter of powdery active carbon was varied between 10 and 3,000 xcexcm to produce various molded catalyst specimens in an attempt of finding an optimal level for the particle size of active carbon particles. As a result, a relatively highly active carbon catalyst could be obtained within a range of average particle diameter of powdery active carbon between 12 and 600 xcexcm as will be discussed hereinafter.
The inventors of the present invention looked into a possible method of effectively improving the water repellency of macropores in order to produce a highly active catalyst by adding PTFE only at a reduced rate. More specifically, the inventors believed that the water repellency of the catalyst can be effectively improved when the surface of powdery active carbon particles and internal macropores is brought into contact with PTFE over a large area by enlarging the area by which PTFE is projected if PTFE is added at a same rate. Thus, the inventors intended to apply shearing force to active carbon particles and PTFE particles when they are mixed together in order to deform PTFE particles and make them adhere to powdery active carbon extensively so that the surface of powdery active carbon particles and internal macropores may be provided with strong water repellency. Then, PTFE particles were added to powdery active carbon at a rate of 0.5 to 30 wt % in the form of PTFE powder or PTFE-dispersed solution and then they were kneaded by means of a kneader, a roll kneading machine, a calender roll or a roll crusher and molded to obtain an active carbon catalyst. The obtained active carbon catalyst was then used in a desulfurization test to find that an active carbon catalyst containing powdery PTFE to a reduced extent operates well same as an active carbon catalyst obtained by simply mixing active carbon particles and PTFE particles and molding the mixture.
Thus, according to the first aspect of the invention, there is provided an active carbon catalyst to be brought into contact with flue gas containing sulfur oxides in order to adsorb and oxidize said sulfur oxides and produce sulfuric acid to be recovered and removed, inter-particulate gaps being formed by combining/molding powdery active carbon to a predetermined profile, the peripheral wall of said gaps being processed for water repellency. Advantageously, an active carbon catalyst according to the invention contains powdery active carbon with an average particle diameter between 12 and 600 xcexcm, preferably between 20 and 200 xcexcm, and fluororesin powder or dispersed solution by 0.5 to 25 wt %, preferably by 1 to 20 wt %, relative to said powdery active carbon and is molded to a predetermined profile after applying shearing force to and kneading the mixture.
In the course of further investigation, the inventors of the present invention came to find that dilute sulfuric acid generated on and in an active carbon catalyst is often not completely discharged from the pores of the catalyst if it has been processed for water repellency. This may be because the dilute sulfuric acid adhering to the surface of catalyst particles is not removed quickly from the reaction vessel and interferes with the possible discharge of dilute sulfuric acid from the pores and the possible contact of flue gas and catalyst particles. If such is the case, the reaction efficiency is reduced as the volume of dilute sulfuric acid increases in the reaction vessel to make it necessary to increase the amount of catalyst within the vessel and baffle any attempt of down-sizing the vessel. Therefore, if the generated dilute sulfuric acid is prevented from remaining on the catalyst and discharged quickly from the reaction vessel, the contact efficiency of flue gas and the catalyst and hence the reaction efficiency thereof can be improved to make it possible to reduce the necessary amount of catalyst.
Thus, according to the second aspect of the invention, there is provided a method of removing flue gas containing at least sulfur dioxide gas, oxygen and moisture by causing it to contact with a catalyst and turn said sulfur dioxide gas into dilute sulfuric acid, said flue gas being made to flow downwardly through the catalyst.
When flue gas is made to flow through a tower filled with active carbon to be brought into contact with flue gas, there arises another problem that a layer of granular active carbon filled in the tower is not economically feasible when used in an flue gas desulfurization plant designed to treat flue gas at a high rate because of a significant pressure loss that occurs there. If the diameter of the tower is increased to reduce the pressure loss, the plant requires large premises and it becomes difficult to evenly and uniformly distribute gas within the tower. In an attempt of reducing the pressure loss of an flue gas desulfurization plant, there have been proposed honeycomb structures including those produced by molding and baking active carbon or some other carbon material, using resin such as petroleum pitch or polypropylene as binder and those made of metal to which active carbon is made to adhere. Some of such structures are commercially available.
However, it is difficult and costly to produce a large honeycomb structure by molding and baking active carbon because of the strain that appears during the baking process. On the other hand, a honeycomb structure made of metal to which active carbon is made to adhere is poorly durable when exposed to flue gas containing corrosive sulfur dioxide gas because the metal of the structure is normally aluminum. Additionally, while the technique of molding a mixture of powdery active carbon and a water-repellent material such as resin, fluororesin in particular, is effective to provide the surface of powdery active carbon with water repellency, a product obtained by extrusion-molding or pressure-molding such a mixture does not provide a sufficient strength for a honeycomb structure. In view of these facts, there is a demand for a method of manufacturing a honeycomb structure containing active carbon and having a sufficient strength without difficulty.
Thus, according to the third aspect of the invention, there is provided a method of manufacturing an active carbon catalyst having a honeycomb structure by kneading a mixture of active carbon and resin and molding the mixture to a plate-like or pillar-like preform and by processing said preform into a honeycomb structure.
Then, there arises still another problem that combustion flue gas of boilers can contain ashes and soot in addition to sulfur oxides such as sulfur dioxide gas depending on the properties of the fuel used in the boiler. This problem also has to be taken into consideration. When a wet system is used for desulfurizing flue gas and sulfur dioxide gas is absorbed by an absorbent solution that is brought into gas/liquid contact with flue gas, ashes and soot will be caught by the absorbent solution along with sulfur dioxide gas so that the both can be removed simultaneously. However, in the case of a dry system, ashes and soot will have to be removed prior to the desulfurization process because, if the solid catalyst is used to catch ashes and soot, the catalyst layer can become clogged by ashes and soot and/or its desulfurization effect can become degraded as the catalyst surface is eroded. While devices for removing ashes and soot include electrostatic precipitaters and gas cleaning towers, the use of such a device is disadvantageous in terms of cost and space. Thus, there is a demand for a desulfurization method for treating flue gas by bringing it into contact with a solid catalyst that does not require the use of an additional dust catching apparatus or, if does, requires only a remarkably down-sized and energy-saving apparatus even if flue gas contains ashes and soot in addition to sulfur oxides.
Thus, according to the fourth aspect of the invention, there is provided a method of simultaneously removing sulfur dioxide gas and ashes and soot contained in flue gas by bringing flue gas containing at least sulfur dioxide gas, oxygen, moisture and ashes and soot into contact with a solid catalyst, the surface of said catalyst being brought into a wet state by dilute sulfuric acid containing at least as part thereof aqueous sulfuric acid solution produced on said catalyst from sulfur dioxide gas, oxygen and moisture contained in flue gas.